Method of generating tooth forms



Aug. 28, 1928. Lessig-)94 E. W. MILLER METHOD OF' GENERATING TOOTH FORMS Filed April l5, 1924 l |H l n @XC S Patented Aug. 28, l1928i.

UNITED STATES y 1,681,994'v rATENr OFFICE..

EDWARD 'W'. MILLER, SPRINGFIELD, VERMONT, ASSIGNOR T0 THE FELLOWS GEAR SEAPER COMPANY, OF SPRINGFIELD, VERMONT, A CORPORATION OF VERMONT.

VHIEIIEOC OF GENERATING TOOTH FORMS. i

Application led April 15, 1924. Serial No. 706,661.

rlhis invention relates to the art of cutting threads in worms, s erews, and the like, by the use of a cutter capable of generating the desired profile in the thread. My primary object is to furnish for this purpose a milling cutter in the form of a-helical gear which not only will remain the same in form after repeated sharpenings, but will require no adg'ustnient as to the distance between its axis and the airis of the work after sharpening, no matter how many times repeated. In other words, my object is to avoid the need of changing the center distance between the cutter and work after sharpening the cutter. nhis object" is accomplished by making the cutter of uniform diameter, and its teeth of uniform thickness, from end to end; and' by forming the cutting ends of its teeth with a top rake or bevel. The cutter is designed to ,i

form threads of the full depth,` or nearly so (when provision is made for a final light iinishing cut) in a single pass along the work. |Clearance between the cutter teeth and the thread is obtained by setting the c'utter with its forward end far enough back from the plane which is both radial to the work and perpendicularv to the cutter axis to avoid contact of any part of the cutter teeth, except their cutting edges, with the sides of the thread; and the top rake of the teeth is made of such value that, when the cutter is set in the position stated, the end faces of the teeth in the middle of theirare of action will be approximately radial to the Work piece. The cutter teeth are also non-symmetrical.

'llhe invention consists in the cutter indicated in the foregoing` general statement and particularly pointed out in the following detailed description and in the claims. It further consists in the new combination of cutter and work above indicated and more particularly pointed out hereinafter. Still further the invention consists in all the novel characteristics of the cutter, not only for the specic purpose above indicated, but for all other purposes to which such cutter may be applicable.

In this specification, and the drawings referred to therein, I have illustrated the embodiment of my invention applied to the specific purpose of cutting the thread of a worm. and I have entitled the cutter as threadgenerating cutter, but it should be undersood that I have not in'tendel by this illus- L trative explanation tc limit the scope of the protection which I claim in any wise short of thefull scope in which the novelty and utility of the invention entitles it to protection.

In the drawings associated herewith;

Figure 1 is a sectional view of the cutter and work piece taken through the axis of the cutter.

Figs. 2, 3, and l are related views showing the projection, in a plane perpendicular 'to the cutter anis, of 'the section of the thread at the cutting plane.

Fig. 5, is a front elevation of the cutter and Work piece, the work piece being shown partly in section on the plane in which the cutting edges of the cutter teeth are located, such plane being indicated by the line l5 in Figure 1;

Fig 6, is a generally similar view, provided for com arison, of a cutter having symmetrical teet acting on a work piece of which the aXis is in the same plane with the operative end of the cutter;

Fig. 7, is a side elevation of my cutter.

Like reference A characters designate the same parts wherever they occur in all the figures.

In my Patent No. 1,461,219, granted July 10, 1923, I have disclosed a complete machine for generating threads by the operation of a gear-shaped cutter having helical teeth, the cutter being both traversed along the work and simultaneously rotated. I refer to this patent for two reasons; first to show an 0perative machine with which my present cutter may be used and, second to illustrate the difference between the new cutter and those heretofore used. On the first point it may be understood without further description that the cutter of the present invention may be mounted on a work spindle, and the latter on a carriage, and given a compound movement of rotation and translation withl respect to the rotating work piece, according to the principles and for the reasons as fully Xplained in said patent.

On the second point, and withreference particularly to Figures 1 and d of the present drawings, it 'will be noted that, whereas previously the helical cutter was arranged with the `plane of its cutting end substantially a plane radial to the work, and its teeth''aa ired olf or tapered from the cutting end in order to provide clearance, now according te the present inventicn, the cutter is made with its diameter uniform and its teeth or the same width and height roin end to end, while the object oit obtaining clearance between the cut-- ter teeth and the thread cut in the work is obtained by setting the cutter back from the plane, perpendicular to the cutter axis, which passes through the axis of the work.

n these drawings l0 represents the cutter having helical teeth il, like those elta helical gear, and l2 represents the work piece, that here illustrated being a worin ot four parallel threads 13. The cutter is made of ma terial suitable tor metal cutting tools. fit one end, that which is the forward or cutting end when in use, the cutter is recessed to a conical surface i4, which forms the necessary top-rake and cutting edges at the ends of its teeth. rlhis is the surface which is ground away when the cutter needs sharpening. The elements et this conical surface make an angle A. with the plane in which the forward extremities oit the cutter teeth lie, such piane being represented by the line l5, and being perpendicular to the anis of the cutter. This plane may be more briefly called the plane of the forward end of the cutter. rllhe line B-B in Figure l represents the radial plane ot the work piece which is perpendicular to the axis of the cutter, and correspondingly is parallel to the plane of the forward end of the cutter. ln the machine with which this cutter is used, the cutter and worli spindle are transverse to one another at suoli a distance apart that the cutter teeth will enter the work to the full depth of the thread, .less the amount (if any) of the thickness of stock to be left for removal by a finishing cut, and the cutter is placed on its spindle at such a distance back from the plane B-B", that only the edges of its teeth will engage the work piece, and the sides of its teeth back of the cutting edges will not rub on the sides of the thread. The angle A of top-rake of lthe cutter teeth is preferably made of such value that the end or top faces of the cutter teeth, when in the middle of the arc of action, become approximately radial to the work. Then the radial plane '16--17 of the work piece, including the nearest point of the cutter, malres'thesaine angle A with the plane B-B. In another mode oit statement, when the cutter is set with the top-rake of the tooth which is in mid cutting position radial to the Work piece, or approximately so, ample clearance is provided between the cutter teeth and the sides of the thread cut in the Work piece.

A more general statement of thisrelation Y between the cutter and the work, 'which applies to the cases of other helically toothed machine elements as well as to worms or screws, may be given on the basis of the fol-' Leanser.

line at such an angle that its teeth clear the L sides of the groove, and this angle may be,

- although it is not necessarily in all cases, equal to the angle A which in Figure l represents the angle of top-rake. Thus my novel cutter and its relation to the worlr may be eirpi'essed in general terms as being a cutter in the form oi a helical gear of unii'erni diameter operatii'ig` at an inclination to the direction of relative cutting movement between itself and the Work.

l will now describe the nature oi the lach ot' symmetry of the cutter teeth, mentioned in the introductory part of this specilication. Such non-symmetry of the cutter teeth is essential for generating symmetrical thread forms because the plane in which the cutting is done is inclined to the axis of the cutter,

while the cutter teeth are generated by revof foi-nis may therefore be most accurately delined in generic terms as corresponding to gear teeth which are conjugate to a rack whereof the teeth correspond to the projection, on a plane perpendicular to the axis of the cutter, of the section of the thread in the cutting plane.

The cutting plane referred to is, iii the case illustrated in Fig. 1, the radial plane of the line 16-17 in which the edges of the cutter teeth lie When such teeth are at the middle of their arc of action (the position of the tooth 11 iii Fig. 5). The section of the thread on this plane, being radial to the work piece, is the profile of the thread. This profile is shown by the dotted outline 18-19- 20-21 in Fig. 2; while the projection of the profile at the cutting plane upon the plane 15 at the end of the cutter and upon any 'other plane perpendicular to the axis of the cutter, as well) is shown by the full line outline 22-23-24-25.

The latter out-line is determined by making projections according to the principles ofdescriptive geometry between Figs. 1 and 2, Figs. 2 and 8. and Figs. 3 and 4. Fig. 4 is an end view of the thread, (as seen from the end of the work) represented a cut off on the plane E, which makes the angle A with a dianietrical plane l-lL-Hpof the work parallel to the cutter axis. Fig. 3 shows the same part of the lthread in side elevation.

.placed in position to be projected on Fig. 2

by lines perpendicular to the cutter axis. The outline O-K--K'-O is the projection in Fig. 3 of the thread profile on plane E. Fig. 4, projected along the lines D and D. .T -J define the line in which the top of the thread intersects a plane, parallel to I"H--H the points O, O on line N (by projection lines F, F), outline 22-23-24-25, is` determined. N and N are projection lines from the top and root of the thread, respectively, at the cutting plane, running parallel to the cutter axis. Y

A cutter made with its tooth curves at one side conjugate to a rack of which the toothfaces have the same inclination to the median line of the tooth as that ot' the line .Q4-25 to the median line of the thread profile, and the curves of the opposite sides conjugate to a rack having faces which bear a like relation to the line 2223, will, when otherwise made as described, and set to cut in a plane making an angle A with the plane B-B, generate and cut threads having the desired profile 18-19-20-21. Since the profiles and projections herein particularly described are composed of approximately straight lines, they correspond substantially to base lracks of the involute system of gearing, and the. cutter teeth are capable of being generated i according'to principles well understood in gearing. Hence, by ascertaining the values of ithe angles' which the Sides 22--23 and 24u-25 respectivelyy make with the median line of the thread prole and then determining the pressure angle and base circle'o'f the conjugate tooth curves, by such principles, it is possible to determine readily and produce accurately the tooth forms necessary to generate a thread of symmetrical profile, or

' one of any predetermined non-symmetrical profile. Of course the absolute values of the angles made by the lines 22-23 and 24-25 with the median linefwill vary in different specific cases both according to the helix angle of the thread, and also to the distance to which the cutter is set back from a line corresponding to the line B-B, but the principles underlving the explanation herein given are applicable to all cases.

Figure 5 illustrates graphically the nonsymmetrical formation of the cutter teeth and their conjugate relationship to-the thread at the cutting plane. The cutter is shown in end elevation 1n this figure, but the part of the work piece which is represented in section and in engagement with the cutter teeth is the projection of the thread profile projected as explained in connection with Figure 2.-

A cutter of the character here described has this great advantage over those in which the sides of the teeth are backed 0E to give clearance, that no change in the distance between the centers of the cutters and work piece need be made as a result of sharpening the cutter. On the contrary-the cutter may be sharpened repeatedly b grinding away its recessed end, as long as tiere is enough of it left to cut the stock, and it will continue to do satisfactory work. After sharpening, the cutter or its spindle need onlybe adjusted lengthwise of its axisenough to compensate for the amount of material removed in sharpening, but no adjustment need be made in the position ot' the spindle itself, and particularly in the distance between the spindle and the axis of the work piece.

It will be apparent from the foregoing explanation that the thread, having in prolile the form of a rack tooth, is essentially a gear tooth form.

. What I claim and desire to secure by Letters Patent is:

l. The method of generating tooth forms which consists in providing a cutter having the form of a helical ear of uniform diameter and uniform wi th of tooth from end l to end, such cutter having a conically recessed end face and cutting edges at the ends of its teeth, applying such cutter to the work with its axis making an acute an le with the momentary direction of relative cutting travel, rotating the Work so as to approach the end of the cutter at the cutting edges thereof, and effecting a relative motion of rolling traverse between the cutter and work lengthwise of the latter.

2. The method of generating threads which consists in providing a cutter having the form of a helical gear of uniform diameter beveled at one end to give top rake to its teeth, applying said cutter to the work piece in a position such -that the top rake of the cutter teeth,-

when, in the middle of their arc of action, is approximately radial to the work piece, rotating the work piece toward the cutter, and effecting a relative rolling movement between the cutter and work piece lengthwise of the work piece.

3. The method of generating threads which consists in applying a cutter having helical teeth which throughout their length are equally distant from the axis of the cutter and are non-symmetrical at opposite sides,

applying said cutter to the work piece with the ends of the cutter teeth in position to cut the work to a predetermined depth and the. plane of the forward end of the cutter set back from and parallel to an axial plane of the work perpendicular to the cutter axis, rotating the work, and rolling the cutter lengthwise of the work in the manner of a gear rollin# in mesh with a rack.

4. The mated of generating threads which consists in providing a cutter having the characteristics of a helical gear with non-` symmetrical opposite tooth faces, which faces are respectively conjugate to the opposite sides o the profiley of a symmetrical thread on a plane inclined tothe plane ot the proiie, applying such a cutter in cutting relation to the Work with its axis perpendicular to a radial plane of the Work and the plane of its forward end displaced to the rear from said radial plane, rotating the Work piece in a direction to cause cutting thereof by the cutter, and effecting a relative rolling travel between the cutter and the Work.

5. The method of generating threads which consists in providing` a cutter having the characteristics of a helical gear of uniform diameter from end to end with a conically beveled forward end face and top rake of its teeth at the cutting ends thereof, forming the opigeaneaa posite :taces of the cutter teeth with nonsymmetrcai face curves which are conjugaterespectively to the tooth faces of racks corresponding to the projections of the thread profile on a plane making an angle equal to the angle of top rake With the plane of the thread profile, applying the cutter to the 'Work in position to cut therein to a prescribed depth and in such relation that the end faces of the cutter teeth are approximately radiai to the Work piece, rotating the Work piece the direction to cause cutting thereonl and producing a relative rolling movement hetWeen the cutter and Werl( piece.

in testimony whereof i have azeti signature. 

